> I've not used Guix, but looking at those slides, the ability to take the build definition of the "hello" program, inherit everything but change the version, looks very interesting.

To do this with any derivation in in nixpkgs, call pkgs.lib.overrideDerivation on it, passing the relevant new attributes. This'll inherit the old attributes, only modifying the ones you supply. This is described in the manual: http://nixos.org/nixos/manual/#sec-customising-packages